loss of 



THE ORIGIN OF MOTIVE POWER. 419 



loss of mechanical effect, as in the friction, impact, 

 cutting, or bending of solids, were alluded to, but 

 especially that which is presented by a fluid in 

 motion. Although in hammering solids, or in 

 forcing solids to slide against one another, 

 it rnay have been supposed that the alterations 

 which the solids experience from such processes 

 constitute effects mechanically equivalent to 

 the work spent, no such explanation can be 

 contemplated for the case of work spent in agitat- 

 ing a fluid. If water in a basin be stirred round 

 and left revolving, after a few minutes it may be 

 observed to have lost all sensible or otherwise 

 discernible signs of motion. Yet it has not com- 

 municated motion to other matter round it ; and 

 it appears as if it has retained no effect whatever 

 from the state of motion in which it had been. It 

 is not tolerable to suppose that its motion can 

 have come to nothing ; and until fourteen years 

 ago confession of ignorance and expectation of 

 light was all that philosophy taught regarding the 

 vast class of natural phenomena, of which the case 

 alluded to is an example. Mayer, in 1842, and 



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